1. Field of the Invention
The present invention relates to a driving method and apparatus for a liquid discharge head for use in printing as well as in manufacturing color filters, thin film transistors, light-emitting devices, DNA devices, and the like.
2. Related Background Art
A liquid discharge apparatus has begun to be used for producing printed materials as well as for a patterning process in manufacturing color filters, thin film transistors, light-emitting devices, DNA devices, and the like.
Photolithography is widely adopted for such an industrial patterning method. However, the photolithography requires many steps and the cost for devices is huge, while providing extremely low material-use efficiency. Meanwhile, offset printing has a limitation on use as an industrial patterning technique due to the precision thereof.
Under the circumstances, a patterning method using a liquid discharge head, which is also called ink jet method, has become popular. The ink jet method allows for direct plotting on a patterning portion, thereby providing extremely high material-use efficiency while requiring a small number of steps, which is a useful patterning technique with low running cost.
Well-known ink jet methods are of the Kyser type described in Japanese Patent Publication No. 53-12138 and of the thermal jet type disclosed in Japanese Patent Publication No. 61-59914 (U.S. Pat. No. 5,754,194).
A shear-mode ink jet method using a piezoelectric ceramic is disclosed in Japanese Patent Application Laid-Open No. 63-247051 (U.S. Pat. No. 4,879,568).
As shown in FIGS. 9A and 9B, an ink jet head (liquid discharge head) 500 incorporating a shear-mode pressure generating device includes a bottom wall 501, a top wall 502, and shear-mode actuator walls 503. Each of the actuator walls 503 is formed of a lower wall 507 which is bonded to the bottom wall 501 and which is polarized in the direction indicated by an arrow 511, and an upper wall 505 which is bonded to the top wall 502 and which is polarized in the direction indicated by an arrow 509. A pair of adjacent actuator walls 503 forms an ink flow path (pressure-applying portion) 506. An air chamber 508 formed of a gap containing no ink is provided between adjacent ink flow paths 506.
An orifice plate 512 having a nozzle 510 is bonded to one end of each ink flow path 506, and electrodes 513 and 514 are provided as metallized layers on both sides of each actuator wall 503. More specifically, each actuator wall 503 is provided with the electrode 514 on the side of the ink flow path 506, and is provided with the electrode 513 on the side of the air chamber 508. The electrodes 513 facing the air chamber 508 are connected to a control circuit 520 for supplying an actuator driving signal, while the electrodes 514 defining the ink flow path 506 are connected to a ground.
A voltage is applied by the control circuit 520 to the electrodes 513 beside the air chambers 508, thus causing the actuator walls 503 to produce shear strain deformation in the direction where the volume of the ink flow paths 506 increases.
For example, as shown in FIG. 10, when a driving voltage is applied to the electrodes 513 beside the air chambers 508, an electric field is generated in the actuator walls 505 and 507 in the directions orthogonal to the respective polarizations as indicated by arrows, thus causing shear strain deformation of the actuator walls 505 and 507 in the direction where the volume of the ink flow path 506 increases. Then, a pressure decreases in the ink flow path 506 including the vicinity of the nozzle 510, so that ink is dispensed from an ink common flow path (not shown) on an ink supply side.
If the hydrodynamic resonant frequency of the inside of the ink flow path 506 is indicated by Fr, an inverse thereof is indicated by Tr (=1/Fr), and the time during which the voltage is applied is set to Tr/2, resonance across the system can be used, thereby making the amount of deformation greater than the original amount obtained as shear strain (non-resonance).
The hydrodynamic resonant frequency Fr can be determined by electric measurement using a well-known impedance measurement device. FIG. 11 shows the relationship between the measurement data obtained by the impedance measurement device (the frequency dependency of impedance) and the hydrodynamic resonant frequency Fr.
After the lapse of the voltage-applying time Tr/2, the voltage applied to the electrodes 513 beside the air chambers 508 is reset to zero. Then, the actuator walls 505 and 507 are deformed so that the ink flow path 506 may contract more than the normal state where the actuator walls 505 and 507 are not deformed and form a straight flow path, thus causing ink to be pressurized. This allows the ink to flow into the nozzles 510, and ink droplets are expelled from the nozzles 510.
In conventional ink ejecting apparatuses of this type, the volume of an ink droplet to be ejected depends upon the shape of an ink flow path, a driving voltage, and the like. Therefore, the shape of an ink flow path and the driving voltage are determined so that desired volume of an ink droplet can be obtained. If an ink jet apparatus is used as an industrial plotter, however, there are demands for high-definition ink jet performance, and for shorter plotting time. In order to shorten the plotting time, it is necessary to reduce the number of pulses required for plotting as much as possible. For higher definition, the pitch of an ink flow path is made narrower, thereby increasing the definition. In order to narrow the pitch of an ink flow path, in view of the limitation of machining, the thickness of a PZT (lead zirconate titanate) wall, which is a piezoelectric ceramic wall and which can change the volume of the ink flow path, must be reduced, and the depth of the ink flow path must also be reduced. This further leads to a limitation of driving voltage. Eventually, a high-definition head reduces the amount of deformation cause by the PZT wall, resulting in a reduced amount of discharge per dot.
On the other hand, Japanese Patent Publication No. 3-30506 (U.S. Pat. No. 4,563,689) describes that an additional pulse is applied before an application of the main pulse in order to determine the top position of ink meniscus in a nozzle, thereby controlling the volume of an ink droplet. By applying an additional pulse, the volume of an ink droplet can be slightly, but not significantly, increased.
Japanese Patent Application Laid-Open No. 2000-280463 describes a proposed method in which the volume of an ink droplet is increased by providing a pulse having a width of 0.30 T to 1.10 T as an additional emission (first emission) pulse before an application of a main emission (second emission) pulse, where T denotes the pulse width of the main emission pulse. In this method, two ink droplets are discharged to form one dot, thus making it possible to increase the volume of an ink droplet by a factor of up to about 1.5. However, it is difficult to further increase the amount of discharge.
As proposed in Japanese Patent Publication No. 6-55513 (U.S. Pat. No. 5,202,659), in order to increase the amount of discharge, a plurality of ink droplets which are sequentially ejected using a resonant frequency are combined in the air to control the volume of the ink droplets. With this approach, it can be expected that the volume of ink droplets sufficiently increases.
In an industrial ink jet apparatus, however, if the distance between a nozzle and a plotted base is extremely shortened in order to increase the deposition precision, a plurality of liquid drops are not combined in the air, but reach the base individually. In other words, there occurs a time lag in ink droplets to be applied for one-dot plotting, causing the reached drops do not form perfect circles, resulting in a failure of deposition precision.